Automatic control for time constant systems



Aug. 12, 1 A. D. BIELEK AUTOMATIC CONTROL FOR TIME CONSTANT SYSTEMS Filed July 27, 1949 3 Sheets-Sheet 1 INVENTOR. ALFR D D. Bl E LEK 4 TTO/ZNEY Aug. 12, 1952 A. D. BIELEK 2,605,965

AUTOMATIC CONTROL FOR TIME CONSTANT SYSTEMS Filed July 27, 1949 3 Sheets-Sheet 2 INVENTOR. ALFRED D Bl E'LEK 77M VI A. D. BIELEK OR TIME CONSTANT SYSTEMS Sheets-Sheet 3 AUTOMATIC CONTROL. F

Filed July 27, 1949 INVENTOR.

ALFR p D. BIELEK BY ATTORNEY Patented Aug. 12, 1952 f. UNI ED STATES PATENT oFFIcE AUTOMATIC CONTROL FOR TIME, I

CONSTANT SYSTEMS Alfred n. Bielek, Nutley, N. J. Application July 27, 1949, Serial No. 107,050

The pres'entinvention relates to an automatic control for time constant systems and more 'particularly to an automatically variable time cons'tant'filter. I j V 1 j Various devices have been proposed to improve the transmission'of music, speech, etc., by reducing the audible distortion of sounds "due to atmospheric disturbances and scratchnoises, e. g. scratch noises of phonograph recordings. Gene'rally,vv such devices deal with filtration means whereby all frequencies above or below an established cut-oil are, Suppressed; The established c'ut ofi, being effective within the audible frequency range, greatly reduces noises at, for example, the higher frequencies within the audible range, with, however, an accompanying suppression of other desirable. sounds above the cut-off point. A manual control isusuallyprovided to produce selective cut-ofi in the audible ran e. A control for such cut-off has heretofore usually been horizontal in performance in that the manual manipulationfof the control means results in a series of sharpcut-ofis, for example at selective jfrequencies, which, when illustrated graphically, indicate a horizontal performance of the cut-oil control system along the audible frequency range. I p v .Accordingto my co-pending app ication Serial Nol' 63,321, now Patent No. 2,541,326. of which this is'a continuation-impart application, suppression of audible noise, instead of being controlled horizontally with a sharp cut-off at the cut-oil frequency-thus, eliminating harmonic content and musical timbre-is controlled vertically from a fixed but manually variable cut-off frequency point beyond which all higher ire-p quencies are attenuated but not: eliminated. Thus,'the harmonic content is retained and the musical timbre is preserved while all audible noise and distortion are effectively suppressed. In the case of musical transcriptions and high quality shellac records, the apparatus reduces scratch noise to a negligible level so that it is extremely diificult to distinguish any surface noise or loss of high response at low amplitude levels even-from a no-suppression condition, While-loss of scratch and other noises are very pronounced. 1

However, my co-pending application refers to a fixed time constant of .22 second for aresistorcapacitor filter network in series with a" variable gain control tube to filter a rectified A. C. signal from'A. C. components so that the variable gain control tube operates entirely on a D. C. poten-' tial. The present application is an improvement on such time constant filter.

2 Claims. (01. 178-44) In automatic volume or noise. control system energized by an A. CJc'urrent and utilizing D; C. control means, it is a usual practice to filter A. C. components in said'D; C. control means by a resistor-capacitor network having an established time constant. The. resistor-capacitor network always has a transmiss'iond'elay tim depending upon its .time'constant. The minimum time constant must be about .l-second long to provide adequate filtering of A. C. components and preferably about .25 second. long- Whena rectified signal level rises and falls slowlsn. e. g.the signal levela's producedby symphonic music, the longer time constant is completely satisfactory because the D. C. output voltage can accurately follow the signal level, ie on a time vs. amplitude basis. However, when the signal level changes abruptly, as evidenced by pizzicato instruments Or passages, wherea singlesolo instrument is suddenly followed by a burst of music from a full symphony orchestra thedelay time, of along time constant filter makes it impossible for-a variable gain contro1 tube in series therewith to accurately follow v theabrupt change in signal level. A time constant of short delay time will not remedy thesituation-because the gate- 30-! tion of the variable gain control tube becomes very noticeable and objectionable, and if short enough,

will produce, serious signal distortion. -The, r,esistor-capacitor networkcannot have a delay time short enough to make the D. C.voltage follow the A. C. signal fast enough so that a rapid rise or decay of signal will essentially be instantaneously followed by the gate, or control tube because of the presence of A. ,0. components which, will produce serious signal distortion. 'Therefore, it follows that a filter of'lon'g time constant is satisfactory for slow musicbut not satisfactory for sudden bursts of music because the musical transient will not, be properly reproduced and considerable afterswish will be experienced by the action ofthe controlfilter of short time constant is unsatisfactory for the same condition because the gate action becomes too noticeable andabrupt-not being fast enough to be undetectable-and serious distortion takes place due to-the presence of of A. C.

components in the D. C. control voltage.

These drawbacks can be remedied by an auto-' matically variable time constant or automatically variable time delay so that the control tube will completely followthe signal under all conditions.

It is one object of the present invention topro-f vide an automatically variable time constant the present inven- 4 filter. It is another object of or gate tube; and a tion to provide a variable delay time in a filter system for filtering A. C. components from rectified A. C. signals. It is a further object of the present invention to provide a variable time constant filter which will under all conditions of changing amplitudes allow a D. C. output voltage to completely follow an A. C. signal across a rectifier. Other objects and advantages of the present invention will become-apparent from the description hereinafter following and the drawings forming part hereof, in which:

Fig. 1 illustrate a schematic diagram of a,

dynamic noise and scratch suppressor,

Fig. 2 illustrates a schematic diagram of an Figs. 3 to 5 illustrate modifications of Fig. I

Referring to Fig. 1, an input signal is applied across the potentiometer l, which is set'to apply a fraction of the input voltage, e. g. of the input voltage, to the electronic amplifier 2. This amplifier'may have a voltage-gain of, for example, ten times the fractioned voltage applied to the'amplifien' Where the potentiometer isset to apply 'Te of the input voltage to the amplifier and the amplifier has a voltage gain of ten, then the output voltage of the amplifier will equal the input voltage applied across the potentiometer, or .will control .the voltage in the apparatus at this stage to zero db gain. However, where the input voltage from one source may differ from that of another, the potentiometer may be re-set so that anamplifier with a voltage gain of ten can bring up a 20,dbsignal to zero db, which is the normal operating level; This control permits great flexibility of operation in regard to input and output voltages. A suitable resistance 3 is provided as a plate load for the amplifier 2, and the capacitor 4 together with a substantially high ohm resistor 5 comprise a high impedance outputtermination for the amplifier 2. The amplifier signal, 'e. g. a zero dbm signal, is transferred from the plate load 31 of the amplifier 2 through the capacitor 6 to the potentiometer 1,

which is actually the control means for dynamic scratch noise suppression: The capacitors 0 and B and'the potentiometer l are a frequency determining network which acts as a variable highpass filter having a low frequency cut-off of about 2500 cycles when the arm of the potentiometer is at a maximum clockwise position. The characteristic of this network is such that amplitude increases with frequency so that essentially full amplitude is transmitted through the network from about 5000 cycles and upward. As the arm of the potentiometer is rotated counter-clockwise, the low frequency cut-off through the network-increases gradually to'about 6000 cycles, while maintaining the frequency pass characteristic above 10,000 cycles per second within -3 db of the maximum position amplitude level. Increasing counter-clockwise rotation beyond approximately 70% merely decreases the overall amplitude of the transmitted frequency band while only slightly altering the low-frequency cut-off point. The control, or potentiometer i, may be manipulated so that a signal input of zero to maximum can be applied through the decoupling capacitor 9 to grid No. l of the amplifier ID. The amplifier I0 is a variable gain tube or'a super-control variable gain amplifier essentially class A in operation with a maximum voltage gain of about ten with a low impedance load. It applies a high frequency feedback signal of varying amplitude through the capacitor H and the resistor 12, which are-elements'of a frequency determining network of high-pass characteristics with a low frequency cut-off of about 2500 C. P. S., which, in conjunction with the network 6, l and 8, produces a. fairly sharp cut-off charaoteristic at the lowest frequency point of the potentiometer l. The feedback signal then goes to the cathode of the amplifier 2, forming a variable degenerative circuit which is most effective at the higher audio frequencies which fall within the scratch or noise distortion bands. The feedback signal is in phase with the A. C.

; voltage across a substantially low impedance I3 provided as the terminal of the cathode of the amplifier 2 in the feedback circuit. The switch [4 is an on-off switch positioned in the feedback voltage line. However, the switch may be positioned at any applicable position, preferably in the feedback circuit.

Simultaneously with the impression of voltage across the potentiometer l to the amplifier 2, a full input voltage is applied to the grid of the amplifier l5,v which is an automatic frequency control. voltage amplifier having a voltage gain of about 10, and the amplified voltage passes through the capacitor It to the potentiometer ll, the capacitor [6 and the potentiometer ll forming a high-pass network to attenuate all frequencies below 1000 C. P. S. The band of passed frequencies is then applied to the grid of the amplifier [8, which, for example, may have a voltage gain of about forty-five. Potentiometer I1 is preset in an individual amplifier system so that an A. F. C. voltage of, for example, -40 volts may be developed across the capacitor IQ for peak amplitudes in the higher frequency region of average commercialrecords. This 40 volts potential is usually suflicient to cause complete cut-off of the amplifier l0. However, before the -40 volts potential reaches the capacitor IE, it is applied to the'grid of said amplifier 18, having a grounded cathode and acting as an A. F. amplifier-diode detector, through the capacitor 20 and the dioderectifier 2|, which causes a rectified D. C. voltage to be developed across the potent-iometer 22, which is avariablepotentiometer so that any potential from 0 to -40 volts A. F. C. voltage can be applied to the amplifier l0, and through the resistor 23 which, in conjunction with the capacitor [9, forms a high frequency filter network Thesystem I9, 22 and 23 is a frequency expansion control enabling a controllable cut-off characteristic for the amplifier Ill so that a zero to a fully open gate action can be obtained on peak passages.

Since the capacitors I6 and 20 and the potentiometers H and 22 attenuate all frequencies below 1000C. P. S., no gate action in the amplifier will occur until sufficient high frequency components are present to mask undesirable noise or, for example, record scratch. The filter network 19 and 23 has a time constant of about .22 second.

I provide a divided network, illustrated by the resistors 24 and 25, so that only a fraction of the A. F. C. voltage is applied to grid No. l of the amplifier [B through the resistor 26. The fraction of the dividing network 24 and 25 may be for example, a five to seven ratio of the A. F. C. voltage, which is a constant fraction, or it may be any other suitable fraction as long as it remains non-variable. Therefore, if the total A. F. C. controlled voltage is l0 volts, the voltage applied'to grid No. i would be '7.1 volts. Since the ratio is constant, a linear rate of rise .of A. F. C. voltages would normally produce a due to the saturation of amplifier 18-, starting at about volts with. complete saturation at volts, a modified logarithmic, control of the feedback voltage to amplifier 2 is. provided. When no signal, is present, no A. F. C. voltageis developed and suppression ismaximumfaccording to the setting of potentiometer l. Asa signal is developed, for example from a record, low levels of music and scratchnoise (even if of'a comparatively high level) will produce only very slight opening of the gate of amplifier [0. As.high content is increased, A. F. '0'. Voltage increases atja linear rate to about 50% open gate," and then, due to logarithmic scaling, the gate opens less rapidly. The screen grid B+ potential of the gate control circuits is returned from the amplifier, I!) through .a decoupling filter formed by the capacitor 2'! and resistor 28'and in conjunction with the load resistor 29 to the B+ potential bus 29a. Amplifier I5 is returned through the load resist-, ance 32 to the B+ bus. .The B+ bus is ,then returned to the B+ potential source through another decoupling filter forined by the capacitor 33 and resistor 3|. turned through the load resistor 33 directly to the B+ potential source- Therefore, according to the combination comprising the high frequencyfeedback system and the A. F. C. voltage control system hereinbefore described, it is apparent that I have provided an electrical feedback type dynamic noise and scratch suppressing apparatus whereby suppressionis controlled vertically from a fixed but manually variable cut-off frequency point beyond which all higher frequencies'are attenuated but not eliminated.

Howeven-a filter system having a time constant of .22 second for the filter network is and 23, as hereinbefore mentioned, although being satisfactory generally for'symphonic music, may be improved considerably'by substituting for the network l3 and'23 anautomatically variable delay instead of a fixed delay. According to Figure 1, the substitution of such an automatically controlled variable delay for the. network I 9 and 23 in a suppressor circuit is illustrated by the location AVTC (automatically variable time constant) shown with broken lines;

' Figure ,2 illustrates particularly the circuit of an automatically variable time constant system located at AVTC' of Figure 1, which incorporates the variable resistor 22 and the usual or main filter network of fixed time constant 19 and 23 having a time constant of, for example, .22 second. The present invention comprises a novel The amplifier l8 circuit is re 6 twice as long as the filter networkconsisting of. resistor 23and its load'capacitor l 9. [Under con ditions of slow rise of, the. unfiltered D..C'. voltage at, for instance, point. A, the D. C.. potential modification of such fixed time constant filter.

which converts the fixed time constant into an automatically variable time constant. Basically, the modification comprises the addition of a plurality of amplifier-reststor-capacitor networks which function as auxiliary circuits andwhich balance each other depending upon the ampli tude and duration of the signal across a rectifier.

Under static conditions, the resistor 34, which is in series with the resistor 22 and which is shunted by capacitor 35, provides a filter network which biases grid I of a thermionic tube 36, of substantially low plate resistance and in series with said filter network, at projected cut-off; therefore very little conduction takes place through the amplifier 3B. The time constant of the resistor capacitor network 34 and 35 is about at, for instance, pointsB and C, in'the absence of circuit elementsnot heretofore described, will rise simultaneously. Point Cis locatedat the input .to the variable gain or gate tube. In. However, when an A. C. signal rises abruptly,'.the unfiltered D. C. voltage at point 'A will followthe. A. CLsign'al instantaneously, but will lag apprecilably'at'the output, or point C,.jdue tethetime constant of the. network [Sand 23.. Considerable time will elapse. before. the capacitor .35 will charge to'the' full value of theD, C. potential applied across it. If no signal was present at point A before the burst, then the capacitor will be completely discharged and no bias will exist on grid I of the amplifier 36. and this amplifier will operate as a short circuit across theresistor 23 for as long a timeas the capacitor .35 does not build up a charge sufficient'to cutofi the amplifier 36. Long before the amplifierf36 is cut off, the capacitor I 9 will already have. been charged up to the full instantaneous peak 'value of the D. C. voltage at point A. However, if .a static D. C..po.tential.exists at point. C and is comparatively high,. e.. .g..20 volts, then the amplifier 36 will be cut ofiibecause the charge existing across the capacitor 35 would be at the projected cut-off for the tube 33.v At such stage, a sudden rise in. D. C. potential, e..,g.. to about 35 volts, the signal at point C would again lag because the voltage at point A is now more'negative than before and the bias applied to grid I. of the amplifier 36 is nolonger sufficient to main-.

tain the amplifier at projected cut-off and the.-

amplifier conducts to charge the capacitor. I9 to essentially the full value existing at pointA, and shortly thereafter thecapacitor35 is charged to the full valueexisting at point A and. a bias will again beapplie'd to the amplifier 36 toplac'e' it at projected cut-ofi. If the signalhas 'a high rate of repetiton of pizzicato effects, the amplifier will successfully follow the fastest musical repetition so long as the-value of capacitor35 is'not too large, e. g. the capacitor .35 should not have an excessively high value and shouldjpreferably have a value, such thatin combination with resistor 34', the time constant will be approximately twice that of the main filter network 19' and 23.

Since the amplifier 36 acts only to charge the capacitor l9, it provides, together. with the re:

sistor 34 and capacitor. 35, a first auxiliary cir-j. cuit and another or second auxiliary circuit is re-'.

quired to provide proper discharge characteristics to eliminate after swish, or the excessive.

work consisting of capacitor 331 and its shunt resistor 39 and its grid I is joined to another loca-" tion in the voltage path of saidaforez'nentioned circuit between resistor 34 and its shunt capacitor 35 through a resistance 40 and its load capacitor 4|. The secondary circuit functions to automatically change the ordinarily fixed time constant of the filter network [9 and 23.to a. variable discharge time constant,so., that a y estates 7,. variation in the signal amplitude is transmitted to the control tube or gate" tube 10; according tothe signal amplitude across the resistor 22 with the avoidance of after swish. For example, with a sudden decrease in signal at point A the voltage difierential will change so that point A is positive with reference to point C. The bias--v ing network consisting of resistor 40 and itsload capacitor 4 I, which, in conjunction with the biasing network of resistor 39 and capacitor 38, normally keeps tube 31 at projected cut-off andhas an extremely short time constant 50 that this voltage differential will act on the gridof tube 31 and the tube 3'! will almost instantly conduct and discharge the capacitor 19 down to the value of D. C. voltagejexisting at point A. Thus'the original fixed time constant of the network l9 and 23 is automatically reduced so that the signal existing at point A is almost instantly transmitted to the gate tube I and the gate tube will successfully follow rapid changes. in the value of D. C. voltage at point A, such voltage changes as occasioned by pizzicato efiects. However, in the case of a full symphony orchestra,it is desirable to have a slower decay characteristic when the music ends abruptly in order to preserve a high order of harmonics and timbre. Forsuch purpose, the biasing network consisting of capacitor 38 and its shunt resistor 39 is such that as the tube 31 is discharging the voltage change across the capacitor 38 tends to increase, inducing a greater biasing voltage across, the resistor 39 and tends to keep the tube 3! at or near projected cut-off, offering considerable opposition to the rapid discharge of the capacitor 13 to ground, due to the existance of a static charge plus an added dynamic charge built up across the network 38 and 39. When the average signal islow, only a very smallstatic charge is built up across capacitor 38 and resistor 39. This, in conjunction with whatever dynamic D. C. charge is built up across the network, will produceonly anegligible increase in decay time, even though musical transients may have a high peak value. When, however, a loud symphonic passage is sustained for any time, a considerable static D. C. biasing voltage is built up across capacitor 38 and resistor 39. This in conjunction with the dynamic decay voltage built up across capacitorv 38 will ofier considerable opposition to the flow of current discharging capacitor 39. This opposition is actually achieved by maintaining a higher bias on tube 37; substantially increasing its plate resistance temporarily over the normal "discharge plate resistance. This action is essentially automatic and will quickly adjust itself to any signal conditions. The gradient of opposition, the time consumed before its initial application, and its duration are a function of the relative values of capacitor 38 and resistor 33 These values can be chosen to suit conditions. This opposition to the conduction of the charge on capacitor l9 to ground preserves the echo, as in the case of abrupt endings, and prevents interaction with the bias voltage developed across network 38 and 39, charging action of tube 33 and.

action in the case of percussion instruments and balances the action of the biasing network 48-4l so that slow action, or a longer time. constant, possible with regard to loud symphonic passages.

' In'the case of noise suppressors using ex-' tremely high suppression characteristics, additional elements may be added as shown by resistors 42 and 44, andcapacitor 43 in Fig. 3.. If noise reduction of the order of 35 db or so is attempted, the decay characteristics must be altered so that under dynamic signal conditions natural reproduction canbe achieved. Resistor 42 and capacitor 43 serve to appreciably increase the decay time above that obtainable with the circuit of Fig. 2..

Depending on size of the resistor 42, a static charge will build up across the capacitor 43, since tube 3'! is only at projected, not actual, cutofi, and a small leakage current will always flow to ground. Iihis charge can be as high as of E0 across capacitor l9. This static charge, in addition to the dynamic charge impressed across capacitor 43 as tube 31 conducts, will greatly delay the complete discharge of capacitor 19. The value or capacitor 43 determines the characteristic decay curve and time delay of the discharge of capacitor 1 9. Addition of resistor 44 as shown, depending on the relative values of resistor 42 and resistor 44, serves to further increase the static voltage acrosscapacitor 43. A maximum Voltage of about A; of the E0 available across capacitor I9 can be so built up, effecting a very great delay to the final discharge, but not the initial part of the discharge curve of capacitor I 9. However, if the total resistive value of resistor 42 and resistor 44 in series is equal to, or less than, the diode load resistance 22, the available peak D. C. voltage output will be reduced.

The net el fect of resistor 42, capacitor 43, and resistor 44, if values are properly chosen is to permit 35 or more db of noise reduction in very soft passages, but pull up the floor" from which the suppressor takes off" to about 20-25 db under usual signal conditions. Thus complete noise reduction and elimination can be achieved, while yet retaining the quality apparent only when using more moderate suppression.

Fig. ,4 shows the addition of an anti-noise modulation device, composed of tube 45, capacitor 46 and resistor 41, as shown in effective parallel combination with capacitor 38 and resistor 39.

Repeated experiments have shown that a piano is the most difficult instrument to cleanly reproduce in a dynamic noise suppressor. The aforementioned AVTC circuits are generally swishproof, but show a definite tendency to produce noise modulation under the influence of strong slowly to moderately repeated, notes, i. e. a sort of fuzzy sound after each single strong note in a piano rendition. This is due to the elements 38 and 39 slowing down the decay rate, normally in an entirely satisfactory manner, but somewhat objectionable for piano, to preserve timbre and echo, usually not present in noticeable degree in piano music. Tube 45 is usually kept at projected cutoif by capacitor 56 and resistor 41. This circuit shunts capacitor 38 and resistor 39, as shown. The time constant of capacitor 46 and resistor 41 are so chosen that most musical variations in level will not occur so rapidly that the bias on tube 45 will be overcome and cause it to conduct. In the case of piano music, the attack and decay ofa single note are so rapid that, due

to the relatively longer time constant, the instantaneous bias value will not be maintained high enough to keep tube 45 cut off. The net result is that it will conduct under the influence ;of a series of strong, singlepiano notes, efieotively shorting out elements 38 and 39 and thus 9 maintaining a very fast decay time for the dura tion of the piano notes, giving effectively much cleaner reproduction without the usual accompanying noise modulation, otherwise noticeable in piano solos. If the piano notes are played at a highly repetitive'rate, or are accompanied by an orchestra, tube 45 will be kept, for the most part, out oif, so that elements 38 and 39 will function in the usual manner.

Fig. illustrates a modification of the invention in the provision of a driver-rectifier system delivering the potential to the input of the AVTC circuit. Element 48 is a driver tube developing sufiicient A. C. voltage to drive the system to the required output. Tube 48 should have a low plate resistance if a high peak current output is desired (low mu triode). Transformer 49 couples the output of 4-8 to the rectifier 50, shown as a full-wave type. The primary impedance of 49 should match the plate resistance of 48 for maximum power transfer. The secondary may have any impedance or turns ratio desired, depending upon output voltage and available peak current desired. Capacitor 5| is the input filter condenser, used here to obtain peak output voltage to load resistor 52, which is preferably of fixed value.

In the RC coupled, shunt rectifier system of e. g. Fig. 1, the minimum charging time of capacitor i9 is limited by the current available at the output of the rectifier, which is exceedingly small, when RC coupled, resulting in a minimum charge time of several milliseconds. For certain applications, it may be desirable to greatly speed up the charge time to the order of a few microseconds. This is accomplished by using the circuit of Fig. 6. A low mu triode 48, coupled through transformer 49 (step-down turns ratio) to rectifier 50, which must have suificient current capacity, so that peak charging currents of nearly any magnitude desired may be produced to make possible practically instantaneous charging of capacitor 9.

It is apparent, therefore, that the time constant of the combination of network herein described is actually variable and automatically controlled. Musical echo and timbre are faithfully preserved to the exclusion of practically every noise or swish which cannot be advantageously eliminated by any other way heretofore known.

What I claim is:

1. A system of time variable capacitance comprising a source of current, an output means, a main filter network electrically connected between said source and said output means and Number consisting of a resistor and a shunt load capacitor of fixed time constant, a first auxiliary circuit comprising a first space discharge device having an anode, cathode and grid and a first biasing filter network connected across said cathode and grid and consisting of a resistor and a load capacitor therefor, said first biasing filter network being electrically connected between said source of current and said main filter resistor so that said first biasing resistor and said main filter resistor are in electrical series, said first space discharge anode being connected to said main filter network at a location in the current path between said main filter network resistor and said main filter network capacitor for charging said capacitor, a second auxiliary circuit comprising a space discharge device having an anode, cathode and grid and a second biasing filter network consisting of a resistor and a load capacitor therefor, said second biasing filter network being connected from the cathode of said second space discharge device to a location in the current path between said main filter network resistor and said main filter network capacitor and in shunt to said main filter network capacitor, said grid of the second auxiliary circuit space discharge device being electrically connected to a location in the current path between the resistor of said first biasing filter network and the said source of current, a third biasing filter network of short time constant consisting of a resistor and a load capacitor therefor and connected in said second auxiliary circuit between said source of current and the grid of said second auxiliary circuit space discharge device.

2. A system of time variable capacitance according to claim 1, comprising a fourth biasing filter network and a series resistor therefor, said fourth biasing filter network consisting of a resistor and. its load capacitor, said fourth biasing filter network being connected from the anode of said second space discharge device to the said series resistor and said series resistor being connected to the current path between said main filter network resistor and said main filter network capacitor.

ALFRED D. BIELEK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date 2,164,939 Pfister July 4, 1939 

